bpf: Check validity of link->type in bpf_link_show_fdinfo()

If a newly-added link type doesn't invoke BPF_LINK_TYPE(), accessing
bpf_link_type_strs[link->type] may result in an out-of-bounds access.

To spot such missed invocations early in the future, checking the
validity of link->type in bpf_link_show_fdinfo() and emitting a warning
when such invocations are missed.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241024013558.1135167-3-houtao@huaweicloud.com

kernel/bpf/syscall.c

@@ -3069,13 +3069,17 @@ static void bpf_link_show_fdinfo(struct seq_file *m, struct file *filp)
 {
 	const struct bpf_link *link = filp->private_data;
 	const struct bpf_prog *prog = link->prog;
+	enum bpf_link_type type = link->type;
 	char prog_tag[sizeof(prog->tag) * 2 + 1] = { };
 
-	seq_printf(m,
-		   "link_type:\t%s\n"
-		   "link_id:\t%u\n",
-		   bpf_link_type_strs[link->type],
-		   link->id);
+	if (type < ARRAY_SIZE(bpf_link_type_strs) && bpf_link_type_strs[type]) {
+		seq_printf(m, "link_type:\t%s\n", bpf_link_type_strs[type]);
+	} else {
+		WARN_ONCE(1, "missing BPF_LINK_TYPE(...) for link type %u\n", type);
+		seq_printf(m, "link_type:\t<%u>\n", type);
+	}
+	seq_printf(m, "link_id:\t%u\n", link->id);
+
 	if (prog) {
 		bin2hex(prog_tag, prog->tag, sizeof(prog->tag));
 		seq_printf(m,